Sounds like quad tendonitis. If the quadricep is too tight and overdeveloped, compressive exercises such as jumping, repeated CKC exercises potentially can irrirate the quad tendon or the suprapatellar tendon.

It could also be suprapatellar tendonitis(remember that bursa about the quad tendon)

Try reducing the exercise to decrease the load, stretch the quadriceps, switch to exercises that integrate the water and use recumbent bike..Let me know if this helps

It sounds like really tight hip flexors. If any of the 3 hip flexors are tight, it can pinch the femoral nerve, causing a burning sensation down the thigh, to the knee, and can be felt any where along the nerve path.

The fact that you have three clients experiencing this tells me that there maybe something in their training programs. After having their physicians clear them, I would regress their their training programs and begin all over again.

Many fitness instructors can get caught up in clients progressing well and move along too quickly. Or can be caught up in getting results and again progress too quickly. Whenever I have a client present unwanted/unintended signs/symptoms, I re-evaluate the program and regress the most likely culprits. And I ask the client about other activities they are currently doing on a regular basis. Often I find that clients are doing concurrent training that is leading to overtraining of the affected area(s).